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The Eustachian tube, or auditory tube, connects the middle ear with the nasopharynx. Its principal functions are ventilation and pressure equalization, clearance of middle-ear secretions, and protection against inappropriate entry of nasopharyngeal material.

Developmentally, its lumen and epithelial lining arise chiefly from the proximal part of the tubotympanic recess, an extension of the first pharyngeal pouch. The expanded distal part contributes to the tympanic cavity. The neighbouring first pharyngeal cleft is associated with the external ear passage and must not be substituted for the pouch as the origin of the auditory tube. Supporting cartilage and connective tissue develop from surrounding mesenchyme. At birth the tube is shorter and more nearly horizontal; craniofacial growth lengthens it and changes its orientation. The developmental account distinguishes formation of the channel from later growth of its surrounding structures.^[ref]

In an adult the auditory tube is roughly 3.5–4 cm long. From its tympanic opening in the anterior wall of the middle ear, it runs forwards, downwards and medially towards its pharyngeal opening in the lateral wall of the nasopharynx, behind the nasal cavity. Its lateral third is bony, lying in the temporal bone; the medial two thirds are chiefly cartilaginous with a membranous part. The narrow junction is the tubal isthmus. The cartilaginous portion is normally collapsed, so an open passage for sound or reflux does not continuously connect pharynx and middle ear.

The bony part is closely related medially to the canal of the internal carotid artery; the canal for the tensor tympani muscle lies above it. The medial cartilaginous part rests towards the skull base. Its pharyngeal end makes the tubal prominence, or torus tubarius, near the soft palate. The tensor veli palatini muscle acts on the cartilaginous and membranous tube and is the main active dilator. The levator veli palatini muscle passes beneath it and participates in the palatal and tubal functional arrangement. Swallowing and yawning intermittently open the tube.^[ref]

Opening permits gas transfer to compensate for a difference between middle-ear and ambient pressure. If the tube remains obstructed, absorption of gas from the cavity can make middle-ear pressure fall; movement of the tympanic membrane and sound transmission then change. Clearance is helped by the mucociliary lining, which carries material towards the nasopharynx. Closure between openings limits reflux and transmission of nasopharyngeal pressure disturbances. A tube that remains persistently open therefore also functions abnormally; more openness is not invariably better.

The physiological point can be demonstrated by observing the tympanic membrane while swallowing during a change of external pressure, or by recording the membrane's mechanical response over a controlled range of ear-canal pressures using tympanometry. These observations examine pressure communication and membrane mobility, not merely whether air can be forcibly driven through a passage. The relatively immature geometry and opening mechanism in young children helps explain their vulnerability to middle-ear fluid retention, but geometry alone is not an exhaustive account of infection risk. The anatomical structures were recognizable in 1866; the modern measurement method should not be mistaken for apparatus supplied by the examination.

The two pneumogastric nerves are the right and left vagus nerves, the tenth cranial nerves: mixed sensory, branchial motor and parasympathetic pathways from the medulla oblongata to the neck, thorax and abdomen. A useful description separates their apparent origin at the surface of the brain from the origins and terminations of their component fibres.

Rootlets emerge in the groove behind the inferior olive, between the glossopharyngeal nerve above and the accessory rootlets below. Branchial motor fibres arise chiefly in the nucleus ambiguus; preganglionic parasympathetic fibres arise in the dorsal motor nucleus of the vagus and, particularly for cardiac control, the nucleus ambiguus. Visceral sensory fibres have their cell bodies chiefly in the inferior vagal ganglion and terminate centrally in the nucleus of the solitary tract; somatic sensory fibres associated with the superior vagal ganglion project to the spinal trigeminal nucleus. Consequently a sensory fibre is not accurately described as originating in a motor nucleus simply because its peripheral nerve emerges nearby.^[ref]

Each vagus nerve leaves the skull through the jugular foramen, with the glossopharyngeal nerve and accessory nerve. It bears the small superior vagal ganglion at the foramen and the larger inferior vagal ganglion below it. Connections include communications with adjacent cranial nerves and the cervical sympathetic nervous system. Traditional descriptions assign fibres joining the vagus nerve from the cranial part of the accessory nerve to the latter peripheral nerve; their modern classification is more nuanced and does not change the peripheral branches being described. In the neck the vagus nerve descends in the carotid sheath, behind and between the internal jugular vein and the internal, then common, carotid artery.

Its meningeal and auricular branches supply small sensory territories of the cranial covering and external ear. Pharyngeal branches contribute to the pharyngeal plexus, supplying much of the soft palate and pharynx; the stylopharyngeus muscle belongs to the glossopharyngeal nerve and the tensor veli palatini muscle to the mandibular division of the trigeminal nerve. The internal branch of the superior laryngeal nerve supplies sensation above the vocal folds; its external branch supplies the cricothyroid muscle. The recurrent laryngeal nerve supplies the other intrinsic laryngeal muscles and sensation below the vocal folds. The right recurrent laryngeal nerve loops beneath the right subclavian artery; the left loops under the aortic arch beside the ligamentum arteriosum. Both return towards the larynx near the groove between trachea and oesophagus.^[ref]

In the thorax, both vagus nerves pass behind the roots of the lungs, contrasting with the anterior course of the phrenic nerves. Cardiac branches join the cardiac plexus; pulmonary branches join the pulmonary plexus. The trunks break up into the oesophageal plexus, then collect principally as an anterior trunk derived from the left vagus nerve and a posterior trunk derived from the right. Through the oesophageal hiatus they reach the stomach; hepatic and coeliac branches distribute to abdominal viscera. Their parasympathetic territory includes foregut and midgut derivatives, extending into the proximal colon, rather than every abdominal and pelvic organ.^[ref] This extensive distribution explains why the level of experimental section is indispensable information.

The main consequences of section can be organized by the interrupted functions:

A discriminating historical experiment would record respiratory movements and heart rate before and after section, distinguish unilateral from bilateral section, and compare an intact airway with an airway bypass. A tracheal cannula can separate obstruction at the glottis from changes in the respiratory rhythm, while the central-versus-peripheral stimulation comparison separates incoming reflex traffic from outgoing motor effects. Later separate-lung recordings confirmed that a change following unilateral section could affect both lungs, supporting a central reflex interpretation rather than simple local paralysis.^[ref] The gastric distinction is also borne out by measurements of persistent acidity after surgical vagotomy, despite major reductions in stimulated gastric acid secretion.^[ref]